1. Field of the Invention
This invention relates to a device for selectively transforming partially polarized light and in particular to a retarder stack for transforming at least partially polarized light for input into various optical devices such as electro-optic modulators, magneto-optic modulators or other optical components.
2. Background of the Related Art
The manipulation or transformation of polarized or partially polarized light is essential in a wide variety of optical systems. Especially with the onset of integrated optics and the processing of optical signals, it is necessary to predictably manipulate the polarization of light before that light proceeds to the next stage in an optical system.
Many optical systems require input light be completely or nearly completely polarized and have a known bandwidth and polarization such as input light from lasers or light emitting diodes (LEDs). It is important, however, to be able to selectively transform portions of relatively wide bandwidth light for eventual use in optical systems. For example, it is desirable to be able to selectively shift a certain band of frequencies within a wide band of frequencies comprising white light.
Color display and color filters are examples of optical systems which utilize wide bandwidth light such as white light to function.
Color display is generally provided by spatial or temporal multiplexing of the additive primary colors, red, green and blue. In a spatial multiplexed display, each color pixel is divided into three subpixels, one for each primary color. Ideally the pixels are small enough compared to the viewing distance from the eye that the colors are spatially integrated into a single full-color image. As a result of subdividing each pixel, the spatial resolution of the display is reduced by a factor of at least three. In temporal multiplexing, colors are sequentially switched between the three primary colors, and if the switching rate is fast enough the eye temporally integrates the three images to form a single full-color image. In both cases, the color filter is typically combined in series with a binary or display capable of generating a gray scale which is spatially aligned and temporally synchronized with the color filter to modulate the intensity of each color. To display white with spatial multiplexing, all three subpixels simultaneously transmit a primary; with temporal multiplexing the three primaries are sequentially transmitted. In either case, at best only one third of the input intensity can be displayed.
In subtractive display, color is produced by stacking three monochrome displays (for example Plummer, U.S. Pat. No. 4,416,514 and Conner et al., U.S. Pat. No. 5,124,818). Polarization components are placed between each display panel, such that each panel ideally independently controls the transmission of an additive primary color. Subtractive displays have the advantage that every pixel is a three-color pixel and that the display does not, in principle, suffer the throughput loss associated with spatial or temporal multiplexing. However, previous implementations generally could not completely independently modulate each color. Additionally, they utilized pleochroic dye polarizers as the only color selective polarization components between each display panel. Due to the poor performance of pleochroic dye polarizers, including poor color contrast, high insertion loss and shallow transition slopes, the benefits of subtractive displays have not before been realized.
There are two basic classes of liquid crystal color switching filters: polarization interferences filters (PIFs) and switched-polarizer-filters (SPFs). The basic unit of an SPF is a stage, consisting of a color polarizer and a two-state neutral polarization switch. This class is intrinsically binary tunable, such that each filter stage permits switching between two colors. Stages are cascaded in order to provide additional output colors. Color polarizers used in SPFs include single retardation films on neutral linear polarizers and pleochroic color polarizing filters. The polarization switch can be a liquid crystal (LC) polarization switch preceding a static polarization analyzer. The switch optimally provides neutral polarization switching. The chromatic nature of the active element degrades performance and is ideally suppressed in an SPF.
Shutters based on color polarizer consisting of a neutral-polarizer followed by a single retarder are well reported in the art (for example in U.S. Pat. No. 4,002,081 to Hilsum, U.S. Pat. No. 4,091,808 to Scheffer and U.S. Pat. No. 4,232,948 to Shanks). While the polarizer/retarder structure can be described as a complementary color polarizer in the sense that it is possible to produce two distinct hues by rotating the polarizer through 90 degrees, using this type of color polarizer in an SPF does not result in saturated colors.
Shutters based on pleochroic color polarizers are also well reported (for example in U.S. Pat. No. 4,582,396 to Bos, U.S. Pat. No. 4,416,514 to Plummer, U.S. Pat. No. 4,758,818 to Vatne and U.S. Pat. No. 5,347,378 to Handschy). Pleochroic color polarizers are films that function as linear polarizers in specific wavelength bands. They are formed by doping a polymer with long-chain pleochroic dyes. Incident white light polarized along one axis is fully transmitted, but is selectively absorbed along the orthogonal axis. For instance, a cyan color polarizer functions as a linear polarizer by absorbing the red along one axis. A color polarizer that passes a primary color (either additive or subtractive) along each axis can be formed as a composite consisting of two films with crossed axes. Colors are typically selected using crossed complementary color (e.g. red/cyan) polarizer films coupled with a switchable polarizer. A full-color device can comprise five polarizing films (one neutral), and two switching means. The resulting structures provide poor overall peak transmission.
The simplest PIFs are essentially two-bean interferometers, where a uniaxial material induces a phase shift between orthogonally polarized field components. Color is generated by interfering these components with an analyzing polarizer. Color switching is accomplished by changing the phase shift between the arms. The most rudimentary color switches comprise a single variable retarding means between neutral polarizers. Single stage devices can also incorporate passive bias retarders with variable birefringence devices. However, these single stage PIFs are incapable of providing saturated color.
PIFS often comprise cascaded filter units in a Lyot structure, each performing a distinct filtering operation to achieve improved selectivity. A polarization analyzer is required between each phase retarder, reducing transmission. Though adequate color saturation is obtained, multiple-stage birefringent filters are by definition incapable of functioning as color polarizer. This is quite simply because color polarizers must transmit both orthogonal polarizations, which does not permit internal polarizers.
Tuning is accomplished by varying the retardance of active elements in each stage, maintaining specific relationships between retardances, in order to shift the pass-band. PIFs use LC elements as variable retarders in order to shift the transmission spectrum. As such, in contrast to SPF, the chromaticity of the active element retardance is not only acceptable, it is often an integral aspect of the design. In PIF designs, an analyzing polarizer is a static component and tuning is accomplished by changing the retardance of the filter elements. When multiple active stages are used, the retardances are typically changed in unison to shift the pass-band, while maintaining the basic design. Variable birefringence PIFs can be tuned to provide peak transmission at any wavelength. By contrast, SPFs do not provide tunable color.
olc filters (olc (1965), J. Opt. Soc. Am. 55:621) provides high finesse spectra using a cascade of identical phase retarders, with complete elimination of internal polarizers. The olc filter is a specific example of a much broader class of filters. In this generalization, Harris et al. Harris et al. 1964), J. Opt. Soc. Am 54:1267) showed that any finite impulse response FIR) filter transmission function can in principle be generated using a stack of properly oriented identical retardation plates. Numerous researchers have used the network synthesis technique, along with standard signal processing methods, to generate FIR filter designs. These designs have focussed on high resolution as opposed to broad pass-bands. Tunability, when mentioned, requires that all retardances are varied in unison.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.
The present invention relates generally to a retarder stack which transforms at least partially polarized light. The retarder stack includes a first retarder having a first retardance and a first orientation with respect to the partially polarized light and a second retarder which has a second retardance and a second orientation with respect to that partially polarized light. The first retarder and second retarder are oriented and have first and second retardances which produce polarization transformed light. In one approach, the at least partially polarized light includes a first spectrum and a second spectrum and the first retarder and second retarder are arranged so that the polarizations of the first spectrum and the second spectrum are not equal. In another orientation, the first retarder and second retarder are arranged with respect to the at least partially polarized light to output a first spectrum and a second spectrum which are complements of each other. In yet another approach, the first retarder and second retarder are arranged and selected to yield a first spectrum and a second spectrum of polarized light which are orthogonal to each other. A particular implementation of this involves a case in which the polarizations of the first and second spectrum are linear and perpendicular to each other. Alternatively, the first and second spectrum can elliptically polarized and still orthogonal to each other in the general sense of orthogonality of polarization states. The light can be visible and/or non visible light.
The retarders can be polymer retarders, liquid crystal polymer retarders, or made of a polymer birefringent material. Additionally, either retarder can be a birefringent crystal such as calcite, quartz, or LiNbO3, or a liquid crystal.
The first and second retardances can be approximately equal or different. If the input light is not even partially polarized, i.e., if the input light is uniformly unpolarized, then a polarizer can be used to provide at least partially polarized light for inputting into the first and second retarders.
The present invention can be used with a color selective polarization modulator and a high brightness color filter or display system. Color separation is accomplished with nearly lossless retarder films, providing high color contrast between transmission and extinction, with steep transition slopes. Each filter stage is a color selective light valve which varies the transmission (or reflection) of one color without modulating the complementary color. A stage can switch between transmitting white (or black) and transmitting a filtered spectrum. Two or more stages can be used in series, each stage independently controlling the transmission of a primary color. In a preferred embodiment each stage can control the analog intensity control of the primary color at each pixel, thus eliminating the need for an external gray-scale pixilated display. One preferred embodiment eliminates internal polarizers between stages, thereby providing a full-color display with only an input polarizing means and an output polarizing means.
The color selective polarization modulator can be for example an electro-optic or magneto-optic modulator having a modulation state of polarization and an isotropic state of polarization, and a retarder stack comprising one or more retarders. The modulation state of polarization is an input polarization for which the transmitted state of polarization depends on the voltage applied to the modulator. The isotropic state of polarization is an input polarization for which the transmitted state of polarization is substantially independent of the voltage applied to the modulator. The retarder stack chromatically preconditions the light such that a first spectrum is placed in the modulation state of the modulator and a second, complementary, spectrum is placed in the isotropic state. The modulator thereby modulates the state of polarization of the first spectrum, but leaves the polarization of the complementary spectrum substantially unmodulated. In a preferred embodiment the spectra are additive and subtractive primary spectra.
A filter is formed by combining the color selective polarization modulator with a polarization analyzer. The polarization analyzer can be a second retarder stack in combination with a neutral polarizer, or it can be a color selective polarizer such as a linear or circular colored polarizing filter, examples of which are pleochroic dye polarizers, and cholesteric liquid crystals, and cholesteric liquid crystal polymers, respectively.
For the case where the polarization analyzer is a second retarder stack in combination with a neutral polarizer, the second retarder stack echoes the first retarder stack, having the same sequence of retardances but in reverse order. The orientation of the second stack is also rotated with respect to the first stack. As a result, in one switching state of the modulator the second stack appears to be crossed with the first stack, undoing the polarization transformation caused by the first stack and, for parallel input and output polarizing means, transmitting white light. For crossed polarizers, the transmission is black. In a second switching state the two stacks are seen as a unit in which the second retarder stack completes the transformation started by the first stack and orthogonally polarizes the first and second spectra. In this state the filter transmits a filtered spectrum.
In the two-stack filter, the polarizers and stacks can be oriented so that the filter is either normally white, i.e. white in the absence of the modulator, or normally filtered. In the former, the action of the modulator is to produce a filtered output, while the latter uses the modulator to generate the white state. In either case, the voltage applied to the modulator controls the xe2x80x9cpresencexe2x80x9d of the compound stack, i.e. the extent to which the two stacks cooperate rather than canceling. If the modulator is capable of analog modulation, the voltage controlled presence of the compound stack is also analog. Analog control of the voltage produces variable throughput of the filtered spectrum.
Each retarder stack has one or more retarders. In order for two stacks to cancel one another in one switching state, if the first stack of retarders have retardances xcex931, xcex932 . . . xcex93N and orientations xcex11, xcex12 . . . xcex1N, then the second retarder stack has retardances xcex93N . . . xcex932, xcex931 and orientations 90xc2x1xcex1N . . . 90xc2x1xcex12, 90xc2x1xcex11. For parallel polarizers the filter is normally white when the second stack retarders are oriented at 90+xcex1N, and it is normally filtered when the second stack retarders are oriented at 90xe2x88x92xcex1N.
Suitable two-stack designs can be generated by choosing the number of retarders N, stepping through a range of values for xcex93N and xcex1N, applying the above rules of retarder orientation to define the second stack, calculating the transmission of the filtered spectrum, and selecting filter designs that produce the desired spectra, typically additive or subtractive primary spectra. Alternatively, certain classes of filter designs can be employed which lend themselves to the white/filtered structure. In particular fan and folded olc filters can be adapted to fit the orientation requirements, as can split-element filters.
In addition to the retarder stacks, additional polarization transforming elements can be included between the input and exit polarizers, for example to resolve compatibility issues between the polarizers and the type of modulator. For polarized light sources, no input polarizer is required. In embodiments having no internal polarizers, the filters can be operated in polarization diversity configurations having polarization separators/combiners for the input and exit polarizers. The filters can also employ reflection-mode designs.
Hybrid filters can be made using the filter of this invention in combination with other active or passive filters. The color filter of this invention can be combined with passive filters, such as retarder based notch filters and dichroic filters for blocking UV, IR or other bands of light. It can be used with other active filters, such as polarization interference filters and switched polarizer filters.
The spectral filters of this invention are particularly useful in the visible spectrum as color filters. They can also be fabricated for use in other wavelength bands for spectroscopy, spectrometry night vision filtering, or wavelength division multiplexing applications. The color filters of this invention can be used in many applications, particularly in the areas of recording and displaying color images. They can be arranged in a multi pixel array, can be spatially or temporally multiplexed, and can be optically addressed.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.